feat(webgl): Add GAL native test harness for visual regression testing

Add a standalone test application that compiles KiCad's actual OPENGL_GAL
against system wxWidgets to generate baseline screenshots for comparing
native OpenGL rendering against WebGL rendering in the browser.

Architecture:
- Compiles 18 KiCad GAL source files from the kicad submodule
- Uses template-based private member accessor (safer than #define private public)
- Generates shader C++ files from GLSL (10 shader pairs for SMAA AA)
- Minimal stubs for KiCad dependencies (PGM_BASE, ADVANCED_CFG, etc.)

Test coverage:
- 11 scenarios testing ~19% of GAL API
- basic-lines, line-widths, circles, arcs, rectangles, polygons
- alpha-blending, transforms, grid-cursor, segments, complex-scene

Key insights documented:
- GAL uses world-to-screen transformation requiring 1:1 pixel mapping
- FBO reading required for clean screenshots on macOS
- Layer depth needed for proper z-ordering in complex scenes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

tests/gal-regression/native/generate_shaders.py

@@ -0,0 +1,102 @@
+#!/usr/bin/env python3
+"""
+Generate C++ shader headers from KiCad GLSL files.
+
+This script converts GLSL shader files to C++ headers compatible with
+KiCad's BUILTIN_SHADERS namespace.
+"""
+
+import os
+import sys
+
+def convert_shader_to_cpp(source_path, var_name):
+    """Convert a shader file to C++ header content."""
+    with open(source_path, 'rb') as f:
+        data = f.read()
+
+    # Convert to hex array
+    hex_values = ', '.join(f'0x{b:02x}' for b in data)
+    hex_values += ', 0x00'  # Null terminate
+
+    array_size = len(data)
+
+    header_content = f"""// Auto-generated shader header from {os.path.basename(source_path)}
+#ifndef {var_name.upper()}_H
+#define {var_name.upper()}_H
+
+#include <string>
+
+namespace KIGFX {{
+namespace BUILTIN_SHADERS {{
+    extern std::string {var_name};
+}}
+}}
+
+#endif // {var_name.upper()}_H
+"""
+
+    cpp_content = f"""// Auto-generated from {os.path.basename(source_path)}
+#include <string>
+#include "{var_name}.h"
+
+namespace KIGFX {{
+namespace BUILTIN_SHADERS {{
+
+static unsigned char {var_name}_bytes[] = {{ {hex_values} }};
+
+std::string {var_name} = std::string(reinterpret_cast<char const*>({var_name}_bytes), {array_size});
+
+}}
+}}
+"""
+
+    return header_content, cpp_content
+
+def main():
+    script_dir = os.path.dirname(os.path.abspath(__file__))
+    kicad_root = os.path.abspath(os.path.join(script_dir, '..', '..', '..', 'kicad'))
+    shaders_dir = os.path.join(kicad_root, 'common', 'gal', 'shaders')
+
+    output_dir = os.path.join(script_dir, 'generated')
+    os.makedirs(output_dir, exist_ok=True)
+
+    shaders = [
+        ('kicad_frag.glsl', 'glsl_kicad_frag'),
+        ('kicad_vert.glsl', 'glsl_kicad_vert'),
+        ('smaa_base.glsl', 'glsl_smaa_base'),
+        ('smaa_pass_1_frag_color.glsl', 'glsl_smaa_pass_1_frag_color'),
+        ('smaa_pass_1_frag_luma.glsl', 'glsl_smaa_pass_1_frag_luma'),
+        ('smaa_pass_1_vert.glsl', 'glsl_smaa_pass_1_vert'),
+        ('smaa_pass_2_frag.glsl', 'glsl_smaa_pass_2_frag'),
+        ('smaa_pass_2_vert.glsl', 'glsl_smaa_pass_2_vert'),
+        ('smaa_pass_3_frag.glsl', 'glsl_smaa_pass_3_frag'),
+        ('smaa_pass_3_vert.glsl', 'glsl_smaa_pass_3_vert'),
+    ]
+
+    generated_cpp_files = []
+
+    for shader_file, var_name in shaders:
+        source_path = os.path.join(shaders_dir, shader_file)
+
+        if not os.path.exists(source_path):
+            print(f"Warning: {source_path} not found, skipping")
+            continue
+
+        header_content, cpp_content = convert_shader_to_cpp(source_path, var_name)
+
+        header_path = os.path.join(output_dir, f'{var_name}.h')
+        cpp_path = os.path.join(output_dir, f'{var_name}.cpp')
+
+        with open(header_path, 'w') as f:
+            f.write(header_content)
+
+        with open(cpp_path, 'w') as f:
+            f.write(cpp_content)
+
+        generated_cpp_files.append(cpp_path)
+        print(f"Generated {var_name}.h and {var_name}.cpp")
+
+    print(f"\nGenerated {len(generated_cpp_files)} shader files in {output_dir}")
+
+if __name__ == '__main__':
+    main()